Balancing ocean cable



July 9, 1935. J. w. MILNOR Re. 19,640

BALANCING OCEAN CABLE Original Filed June 28, 1932 FIG. 6

LI R2 L2 INVENTOR 21- 1 J. w. MILNOR F v 3 hu /3m A TORNEV Reissued July9, 1935 PATENT OFFICE 19,640 BALANCING OCEAN CABLE Joseph W.

Milnor, Maple wood, N. 1., assignor to The Western Union TelegraphCompany, New

York, N. Y., a corporat Original No. 1,941,102, dated 619,793, June 28,

Serial No.

for reissue March 13, 1935,

12 Claims.

This invention relates to a submarine cable system and to a method ofand means for balancing a long submarine telegraph cable by artificialline networks.

It relates particularly to the balancing of a non-loaded submarine cablefor duplex operation but is applicable to any type of long cable, eitherloaded or non-loaded.

In my prior United States Patent No. 1,519,870, granted December 16,1924, I disclosed a simple resistance-capacity artificial line networkfor balancing non-loaded cables which can be adjusted to accuratelysimulate the resistance, capacity and inductance of a cable, providedthese factors do not vary with frequency. Actually they do vary withfrequency in the real cable even at slow signaling speeds and inpractice such variations with frequency may be approximatelysimulated-in the network of the aforesaid patent, by introducing certainirregularities in the network to provide a compromise adjustment. Thereis. however, a definite limit to the accuracy of the balance which maybe obtained with this and similar types of artificial line networks,particularly for the higher frequency components of the telegraphsignals.

In a later patent granted jointly to myself and W. D. Cannon, No.1,815,629, dated July 21, 1931, there is shown a number of artificialline networks involving the use of inductance coils. These networks weredeveloped to overcome the limitations of the networks of said earlierpatent and if properly proportioned they have the characteristics of asmooth line, accurately matching the actual cable both in impedance andpropagation constant throughout a wide range of frequency. Thesenetworks are somewhat complicated and expensive, however, and therefinement thereof are in many cases not required.

One of the objects of the present invention is, therefore, to produce anartificial line network whichwill enablea materially improved duplexbalance to be obtained, as compared to that ohtainable with the ordinaryform of resistance-capacity networks, but with small increase in thecomplexity or expense of the balancing equipment.

Another object is to produce an artificial line network of simple formwhich will provide an accurate balance for the impedance of the cable.simulating the variations in the resistance and inductance of the cablewith frequency, over the signaling range.

Other objects and advantages of the invention will hereinafter appear.

ion of New York December 26, 1933, 1932. Application Serial No. 11,181

The current flow in a submarine cable system returns in the armor wiresand in the water surrounding the cable. The impedance of a nonloadedsubmarine cable of approximately uniform structure throughout its lengthinvolves principally the direct current resistance and the fixedinductance of the cable, the cable capacity, the alternating currentresistance and inductance of the cable which are variable withfrequency, and of lesser importance, the dielectric absorption. Thevariation of the resistance and induc tance with frequency may bedesignated the "sea return effect and is due to the fact that while thevery low frequency components of the return current spread out through arelatively wide area of the sea water, the higher frequency componentsof the current are crowded into a smaller area, current above a certainfrequency being practically all confined to the armor wires of the cableas a return path.

The various networks described hereinafter, when elements of the propervalue are used therein, accurately simulate the "sea return effect" andother electrical properties of non-loaded cables and with somewhat lessaccuracy the propagation constant. In the theory subsequently developed,it is assumed that the cable capacity is constant with frequency andthat the dielectric losses are negligible. In practice these losses arenot entirely negligible but they are small and may be compensated byadding special networks to balance the same, as shown in the aforesaidPatent No. 1,815,629, or they may be balanced by making slightalterations in the values of the elements of these networks. If thecable is of uniform structure throughout its length, that is, if thereare no large irregularities at which reflections may take place, thepropagation constant may be neglected without serious error.

In accordance with the present invention 1' prefer to employ a fiveelement network which, in one embodiment, may consist entirely ofcapacity and resistance, or in another embodiment may possess seriesinductance. While the various elements of the network interact toproduce the duplex balance, it may be stated in general that the directcurrent resistance is balanced by a series resistance element in theartificial line, the capacity is mainly compensated by a plurality ofshunt capacity elements in combination, and the fixed inductance and thevariable resistance and inductance are simulated by the interaction ofthe series resistance and the plurality of shunt paths, the shunt pathsbeing the more important factor in this simulation. In some cases afixed inductance may be included in the series path. These generalproperties can be best expressed in their specific relations by means ofmathematb cal formulae, which will be developed in this specification,in connection with the accompanying drawing, in which:

Figure 1 is a diagrammatic illustration'of a cable terminal providedwith an artificial line embodying my invention;

Figures 2 to 5, inclusive, illustrate alternate forms of networks forsimulating the characteristics of ocean cables, including the sea returneffects; and

Figure 6 illustrates a basic type of network used heretofore forbalancing ocean cables.

Referring first to Figure 1, I have shown a conventional submarine cableIt] terminating in the usual manner for duplex working, the cable beingconnected to the arms of a Wheatstone bridge provided with condensers Hand I2. A transmitter I3 is connected between the junction of the bridgearms and ground, and the receiving apparatus I 4 is connected acrossconjugate terminals of the bridge. The artificial line AL provides abalance for the cable so as to prevent transmitted signals fromaffecting the local receiver.

The artificial line is divided into a number of sections '6, l1, I8,etc., each simulating the electrical properties of a definite length ofcable. To facilitate the adjustment of the elements of the artificialline networks, I divide the series resistance of each section into twoparts, Re, and Rh, the former being on the line side and the latter onthe ground side of the artificial line. The capacity of the artificialline is provided by two condensers C2 and C3 connected between points inthe opposite resistances Ra and Rh. Resistances R2 and R3 are includedin series with the condensers C2 and C3 respectively. One of thecondensers, for instance C3 is of relatively large capacity and incombination with the smaller capacity C2 balances the capacity of thecable. The two shunt paths C2, R2 and C3, R3 and the resistances Ra andRe, by their interaction serve to balance the fixed inductance, and alsothat part of the resistance and inductance which vary with frequency.

The dielectric losses of the cable are not completely balanced by thenetwork shown but may be substantially balanced by adding additionalresistance-capacity shunt paths as described in Patent No. 1,815,629,referred to above.

The network of Figure 2 is similar to that of Figure 1 except that allof the series resistance has been included in the line side. Thisarrangement is somewhat simpler than the network of Figure 1 butordinarily is less desirable in operating practice.

It should be noted that the networks of Figures 1 and 2, despite thefact that they balance with sufficient accuracy the variations ofinductance and resistance of the cable, within the frequency rangeinvolved, do not include any inductance coils.

In Figure 3 the resistance shown in series with one of the shuntcondensers in the previous networks, has been omitted and an inductancecoil L1 has been inserted serially in the artificial line to assist inbalancing the fixed inductance of the cable. In this modification theshunt capacity and capacity-resistance paths are connected across thetwo sides of the artificial line at approximately the midway point ofthe coil L1, the two halves of which are mutually coupled. The two shuntpaths in combination again balance the major portion of the variableinductance and resistance of the cable.

In Figure 4 a similar network is shown in which separate inductancecoils are used, the sum of their inductances being equal to theinductance L1 of Figure 3.

Figure 5 shows a network similar to that of Figure 3 except that theseries resistance and inductance are included in opposite sides of theartificial line. This arrangement may provide a more convenientconstruction in some instances but does not alter the electricalcharacteristics of the network.

For the purpose of illustration, complete calculations for the networksof Figures 3, 4 and 5 are given below to show mathematically the closesimulation of the real cable properties with these general types ofnetworks, and to give the equations for obtaining the values of theelements of the networks in terms of measured properties of the cable.

The complete expression for the characteristic or surge impedance of anon-loaded cable, includ- Z R+jwL G+jwC in which the expression R+fwLrepresents the linear resistance and reactance of the copper conductorand sea return path, and the expression represents the leakage andcapacity reactance of the insulating material where R=resistanceL=inductance G=leakage C=capacity w=21r times the frequency Aspreviously stated both R and L vary somewhat with frequency.

In Figure 6 I have shown an artificial line network copied from theprior Patent No. 1,815,629, the impedance of whic using the symbolsshown on the drawing is:

: jwL R This network was shown in said patent to closely approximate theR+iwL term in the equation for surge impedance. Equation (I) may berewritten as follows:

Substituting the value of Z; in the equation for the surge impedance,the equation becomes, if leakage is omitted,

+J' f) Z 1/ o+jwn jw (3) Equation (3) is a close approximation of theimpedance characteristics of a submarine cable providing the constantsA, B, C, D and R2 are correctly chosen. These values can be computedfrom the quantities of the network of Fig. 6, which were originallybased upon the measured cable parameters. It is obvious that anynetwork, the impedance of which can be expressed in the form of Equation(3) can also be made substan: tially equivalent to the cable.

It will now be shown that the impedance of the network of Fig. 3, forinstance, is of the form of Equation (3).

The impedance Z2 of the network of Figure 3 is given by the equation:

This group of four Equations 9, 10, it and 12 gives the values of theresistances, inductances and capacities to build the network of Figure3. The valueshowever, are expressed in terms of A, B, D, C and R1, ofwhich the last two terms can be measured directly from the cable, butthe synthetic parameters A, B and D must be determined from Equation (1)which, it has been pointed out, represents the impedance of a basicnetwork equivalent to certain properties of the cable. With these fiveparameters, however, the values of L1, R1, C4, C5 and R4 forconstructing the network of Figure 3 can be determined. Therefore, it ispossible to balance the impedance of a cable with an artificial lineconstituted of sections of the type of Fig. 3, and a method forcomputing the values has been given.

As has been previously pointed out, the networks of Figs. 4 and 5 areequivalent to that of Fig. 3, embodying only a rearrangement of some ofthe elements in the circuit. Since the same symbols are used, Equations9, 10, l1 and 12 are directly applicable.

The networks shown in Figures 1 and 2 can be by an equation of similarform to (4) from which the values M, N, P and expressed Equation n. n/jw( i+j 5ciRi 1 If this network be designed to match a section of cableof length m, the above equation becomes:

94121111 2 (L l (1111 mm i er,

jwm(C5+ 4+j s 4 i) but if the sections are made quite small, the termsin m can be neglected; hence:

In 9 i J 5+ -i+j 5 i 4) J liklt isi l .5 4 Z2 (1+j P) jwc The terms M,N, P and C correspond to the terms A, B, D, and C respectively ofequation (3) and have the following values L M: -i l R11 C4R4L1 9 s& PCH- Cs (7) C:C4+C5 Equation (4), it is seen is identical in form withEquation (3) and therefore possesses impedance characteristicssimulating those of the cable if M, N and P are made equal to A, B andD, respectively. If the notations A, B and D be substituted for M, N andP in the above equations and the four equations solved simultaneously,the values for C4, C5, R4 and L1 will be found to be C can be determinedin terms of capacity, resistance and inductance and from which thevalues for the elements Rl Ra+Rb R2, R3 C2 and C3, can be computed.These equations have not been developed in detail herein, but the valuesof M, N, P and C and of R2, R3, C2 and C3 as determined from suchequations for the networks of Figures 1 and 2 are as follows:

Obviously various other arrangements of networks can be devisedembodying the principles described herein, which will closely simulatethe electrical properties of the cable, and, therefore, I do not desireto be limited to the particular forms shown and described butcontemplate all types of elemental networks utilizing the principles setforth.

What I claim is:

1. An artificial line network adapted to balance a submarine cable overa range of frequencies, comprising a plurality of sections, each sectionincluding series resistance and a plurality of shunt paths, said shuntpaths in combination with the series path serving to balance theresistance and inductance of the cable together with their variationswith frequency.

2. An artificial line network adapted to balance a section of submarinecable over a range of frequencies comprising series resistance and aplurality of shunt paths one of which includes a capacity element forbalancing the major portion of the cable capacity and the other of whichcontains series capacity and resistance and adjusted to balance incombination with the series path and the first shunt path the variationsof the resistance and inductance of the cable section with frequency.

3. An artificial line network adapted to balance a section of submarinecable over a range of frequencies comprising series resistance, and aplurality of shunt paths, one of which includes a capacity element forbalancing the major portion of the cable capacity and another of. whichcontains series capacity and resistance and adjusted in combination tobalance primarily the variations of the resistance and inductance of thecable section with frequency, and an additional element for balancingthe major portion of the fixed inductance of the cable section.

4. An artificial line network for balancing a section of submarine cableover a range of frequencies comprising series resistance and a pluralityof capacity-resistance shunt paths, adjusted in combination with theseries path to compensate for the capacity, constant resistance andinductance, and the frequency variable resistance and inductance of thesection.

5. An artificial line network for balancing a section of ocean cablecomprising five elements consisting of resistance and capacity only,said elements being arranged and adjusted to balance the variations inimpedance of the cable with frequency.

6. An artificial line network for balancing a section of ocean cableover a range of frequencies comprising a series resistance for balancingthe major portion of the fixed resistance of the cable section and aplurality of shunt paths including capacity and resistance only forbalancing the capacity and the major portion of the constant inductanceand frequency variable inductance and resistance of the cable section.

7. An artificial line network for balancing a section of ocean cableover a range of frequencies comprising a series resistance for balancingthe fixed resistance of the cable section and shunt paths includingcapacity and resistance only for balancing the frequency variableinductance and resistance of the section.

8. An artificial line network adapted to balance a section of oceancable over a range of frequency comprising series resistance and aplurality of shunt paths, one of said shunt paths balancing the majorportion of the capacity of the cable section and the two shunt paths incombination with the series path balancing the variations of theresistance and inductance of the cable section with frequency.

9. An artificial line network for balancing a section of submarine cableover a range of frequencies comprising five elements including seriesresistance and inductance adjusted to balance the major portion of thefixed resistance and inductance of the cable section, and a plurality ofshunt paths, one containing capacity of a value substantially equal tothe capacity of the cable section and the other consisting of capacityand resistance adjusted to balance the remainder of the cable capacityand by interaction with said first path and the series path to balancethe variations of the resistance and inductance of the section withfrequency.

10. In an artificial line adapted to balance a submarine cable whichpossesses capacity, and also resistance and inductance which vary over asignaling frequency range, the combination of a series resistance pathand a plurality of shunt paths, the shunt paths being adapted to balancethe capacity and also the major portion of the frequency variableinductance and resistance of the cable.

11. An artificial line network adapted to balance a section of oceancable over a range of frequency comprising series inductance with aplurality of shunt paths connected to the approximate mid-point of saidinductance, said shunt paths being adapted to simulate the capacity andsubstantially all of the variable resistance and inductance of thecable.

12. In an artificial line adapted to simulate a section of submarinecable which possesses capacity, inductance, and resistance and alsoresistance and inductance which vary over a signaling range, a seriespath and a plurality of shunt paths containing capacity, with resistancein at least one path, the sum of the capacities simulating the capacityof the cable, while the shunt paths in combination simulatesubstantially all of the variable inductance and resistance of thecable.

JOSEPH W. MILNOR.

